1. Field of the Invention
The present invention relates to a radio receiving apparatus and, particularly, to a technique of receiving a frequency-hopping communication signal.
2. Description of Related Art
In the field of radio communication, a multicarrier transmission system is in the limelight as a technique of suppressing the effect of frequency selective fading. Because the multicarrier transmission system transmits data by dividing it into a plurality of carries with different frequencies, the frequency band of each carrier is narrow, so that it is less subject to the effect of frequency selective fading.
For example, in the orthogonal frequency-division multiplexing (OFDM), which is one of the multicarrier transmission system, the frequency of each carrier is set so that the carriers are orthogonal to each other within a symbol space. When transmitting data, the system performs the serial-to-parallel conversion of a serial signal in each symbol period to a plurality of pieces of data, and allocates the plurality of pieces of data respectively to a plurality of carriers. It then performs the modulation of the amplitude and the phase of each carrier. After that, the system performs the inverse FFT transform of the plurality of carriers so as to transform a frequency-based signal into a time-based signal while maintaining the orthogonality of each carrier which is exhibited in the frequency base, and finally transmits it. When receiving data, the reverse processes are performed. Specifically, the system performs the FFT transform so as to transform a time-based signal into a frequency-based signal, then performs the demodulation of each carrier corresponding to the modulation method, and finally performs the parallel-to-serial conversion to thereby obtain the original serial signal.
In the multiband-orthogonal frequency-division multiplexing (MB-OFDM) which is known as the ultra wide band (UWB) that employs the OFDM system, the frequency band of 3.1 to 10.6 GHz is divided into 14 bands at the frequency band of 528 MHz each. The 14 bands are then grouped into 5 band groups, each group including 3 or 2 bands, and an OFDM signal is transmitted with frequency hopping according to a time frequency code in each band group (cf. “High Rate Ultra Wideband PHY and MAC Standard”, Ecma international standard ECMA-368 1st Edition, December 2005, pp. 7, 14-16).
FIG. 7 shows an example of a signal which is received by a receiving apparatus in the MB-OFDM communication system. The upper part of FIG. 7 shows the receiving signal in the received order, and the lower part of FIG. 7 shows the receiving signal in the upper part sorted into frequency bands. Note that, in the symbol “Sab” which represents an OFDM symbol, “a” indicates a frequency band, and “b” indicates the sequential number of an OFDM symbol in the frequency band. In the example described below, the number of frequency bands is three, the center frequencies of the three frequency bands are f1, f2 and f3, and the frequency hopping is repeated in the order of f1→f2→f3 within the band group.
As shown in FIG. 7, the first OFDM symbol S11 in the frequency band 1 with the center frequency f1 is received firstly. Next, the first OFDM symbol S21 in the frequency band 2 with the center frequency f2 is received. Then, the first OFDM symbol S31 in the frequency band 3 with the center frequency f3 is received. After that, the OFDM symbols S12, S22, S32 and so on are received sequentially. In this manner, a communication signal is transmitted with hopping among three frequency bands. Further, a zero-padded-suffix (ZPS) with a zero value is added to the end of each OFDM symbol as shown in FIG. 7, thereby keeping a guard interval and a time for switching center frequencies. In this description, the combination of an OFDM symbol and ZPS is collectively referred to as a symbol.
FIG. 8 shows a direct-conversion receiver as an example of a receiving apparatus of a radio communication system. The receiver in FIG. 8 is such that a symbol is assigned to each element in FIG. 1(a) of Satoshi Tanaka, Taizo Yamawaki, Kumiko Takikawa, Norio Hayashi, Ikuo Ohno, Tetsuya Wakuta, Satoru Takahashi, Masumi Kasahara, Bob Henshaw, “GSM/DCS1800 Dual Band Direct-Conversion Transceiver IC with a DC Offset Calibration System”, Solid-State Circuits Conference, 2001. ESSCIRC 2001. Proceedings of the 27th European 18-20 Sep. 2001, pp. 494-497), for convenience of description. As shown in FIG. 8, a received signal RF Input is amplified by an amplifier 1 and supplied to two processing circuits. One of the two processing circuits includes a mixer 2, a low-pass filter 3 and an amplifier 4, and the other one includes a mixer 5, a low-pass filter 6 and an amplifier 7. The mixer 2 and the mixer 5 respectively receive local signals whose phases are different from each other by 90 degrees. Each mixer multiplies a communication signal by a local signal, so that frequency-conversion from an RF signal into a baseband signal is done. The low-pass filter 3 extracts low frequencies from the output of the mixer 2 and outputs it to the amplifier 4. The amplifier 4 amplifies the signal and outputs it to the baseband processing circuit 8. Likewise, the amplifier 7 amplifiers the low frequencies which are extracted from the output of the mixer 5 and outputs the amplified signal to the baseband processing circuit 8. The two signals which are input to the baseband processing circuit 8 are I-axis and Q-axis baseband signals. The baseband processing circuit 8 performs AD conversion, FFT transform or the like on those signals, thereby obtaining original information.
Because the above-described direct-conversion receiver directly frequency-converts a received signal into a baseband signal by multiplying the received signal by a local signal, it allows a receiver to have a wider band easier, thus increasing the flexibility of the configuration of the receiver. However, because the receiver of this system uses a local signal having the same frequency as the center frequency of a received signal, if the local signal leaks out and returns to the mixer, DC offset is generated in the output of the mixer due to the local signal's self mixing (LO self mixing). The DC offset is superimposed on a baseband signal to enlarge the voltage amplitude width of the signal, which causes signal distortion to occur due to saturation in the low-pass filter or the amplifier in the subsequent stage, leading to signal degradation. Further, if the voltage amplitude width of the signal which is input to an analog-to-digital converter (ADC) in an input unit of the baseband processing circuit 8 increases because of the above-described enlargement of the signal voltage amplitude width, it becomes problematic because it leads increase of the number of required bits for the ADC.
Because the MB-OFDM communication system transmits a signal with frequency hopping, it is necessary to switch the frequency of a local signal at a receiving side in synchronization with the frequency hopping. If the frequency of a local signal changes, the amount of DC offset which occurs in the mixer output changes accordingly due to a change in the amount of local signal leakage to the mixer input or the like. As a result, the DC offset in the mixer output in the MB-OFDM receiver changes step-like as shown in FIG. 9. Since the frequency of the frequency hopping is 3.2 MHz, which is the same as a symbol rate, the cycle of a change of DC offset is 1/3.2 MHz=312.5 ns.
A general method of removing DC offset in a baseband pass is to insert a capacitor in series to the output of a mixer which multiplies a received signal by a local signal frequency. This method cuts off DC component by a high-pass filter (HPF) which is formed by a capacitor C and a resistor R to thereby remove DC offset. The cutoff frequency of the HPF is 1/(2 pCR), and the convergence time of step response is 2 pCR. R indicates an input resistance of an amplifier or a filter which is connected to another end of the capacitor C that is connected to the mixer output.
Because the subcarrier frequency in the MB-OFDM communication is 4.125 MHz, it is necessary to pass up to 4.125 MHz in a receiver. If the cutoff frequency of the HPF is 4.125 MHz, the convergence time of step response is as long as 242 ns, so that a large part of the time in an OFDM symbol involves the step response. If the cutoff frequency is further reduced in order to make sure to pass a signal of 4.125 MHz, the response convergence time becomes so long that the DC offset does not become zero within one symbol but affects the next symbols.
Further, the MB-OFDM communication system performs frequency hopping for each symbol, and the switching time of the frequency is 9.47 ns. It is difficult to remove DC offset within such a short switching time. For example, according to the technique disclosed in Japanese Unexamined Patent Application Publication No. 2004-172693 (Ono et al), the DC offset correction takes 20 μs, and it is thus incapable of performing high-speed DC offset correction, which is required when receiving a communication signal in the MB-OFDM communication system.
As an approach to the above issue, Japanese Unexamined Patent Application Publication No. 2006-203686 (Iida) discloses a technique of placing an RC circuit for each of frequency bands to be hopped and controls on/off of the switch in synchronization with frequency hopping, as shown in FIG. 10. According to the technique, a capacitor #1 holds electric charge and stops its step response just before frequency hopping from a frequency band #1 to another frequency band #2, and when hopping is performed to the frequency band #1 again, the capacitor #1 continues the step response. After repeating such a capacitor switching operation corresponding to the frequency hopping, the charge or discharge of the capacitor #1 ends to enter the steady state. FIG. 11 shows this process. The input voltage in the upper part of FIG. 11 is DC offset which occurs in a signal which is frequency-converted by a mixer at the frequency band #1, for example, and the lower part of FIG. 11 shows the voltage at the output end of the capacitor #1, which corresponds to the input voltage in the upper part. As shown in FIG. 11, after several symbols in the corresponding frequency bands (which is three symbols in the example of FIG. 11), the voltage at the output end of the capacitor becomes 0, and the DC offset is removed. Specifically, the technique performs the process of removing DC offset independently for each hopped frequency band, so that the transient step response which occurs in a certain frequency band does not affect the next hopped frequency band.
Further, H, Aytur et al. “A Fully Integrated UWB PHY in 0.13 μm CMOS”, ISSCC Dig.Tech.Papers, pp. 124-125, February 2006 discloses a technique of placing a demodulator for each hopped frequency and switching demodulators according to the frequency of a symbol. The technique places a current digital-to-analog converter (IDAC) for correcting DC offset for each demodulator, adjusts each IDAC so that to perform an appropriate correction for each frequency bands. The technique then selects a demodulator and an IDAC output for DC offset correction according to the frequency hopping, so that voltage variation due to hopping does not occur in the output of the demodulator.
As described above, the technique taught by Iida requires several symbols until the charge of the capacitor completes, or until the DC offset is removed completely, for each frequency band to be hopped. Thus, at the start of hopping reception, DC offset remains in a baseband output over several symbols for each frequency band.
The technique taught by H, Aytur et al. requires the adjustment of IDAC and thus needs a time for the adjustment, which prevents prompt startup. Further, the technique requires a circuit and a sequence for adjusting IDAC, which causes an increase in circuit size.